EP0932850A1

EP0932850A1 - Multicolor electrophotographic printing device with bipolar toner

Info

Publication number: EP0932850A1
Application number: EP97947694A
Authority: EP
Inventors: Volkhard Maess; Martin Schleusener
Original assignee: Oce Printing Systems GmbH and Co KG
Current assignee: Canon Production Printing Germany GmbH and Co KG
Priority date: 1996-10-17
Filing date: 1997-10-16
Publication date: 1999-08-04
Anticipated expiration: 2017-10-16
Also published as: WO1998018050A1; DE19781177D2; DE59707809D1; US6278855B1; EP0932850B1

Abstract

The present invention pertains to an electrophotographic device for printing a multicoloured image, comprising the following steps: a) surface elements of a photoconductor layer are loaded on an initial potential (step S1); b) the surface elements are exposed to various degrees (step S2); c) the surface elements are processed with positive particles of a third colour (step S3); d) the surface elements are processed with negative particles of a fourth colour (step S4); 3) the surface elements are arranged closed by a light source (step S6); f) the surface elements are processed with the negative particles of a third colour (step S7).

Description

description

Process for multicolor electrophotographic printing with toners of two polarities

The invention relates to a method for electrophotographic printing of a print image with several colors on an image carrier.

A method for electrophotographic printing with two colors is known from US Pat. No. 4,078,929. This process is also called the "tri-level process". In this method, a printed image contains at least a first picture element of a first color and at least a third picture element of a third color. A second picture element of the printed image has a background color of the image carrier, so that no toner particles have to be applied.

The first picture element is assigned to a first surface element of a photoconductor layer. The photoconductor layer and an electrode layer carrying a predetermined reference potential are contained in a light-sensitive layer system. The

The reference potential is usually the zero potential. Likewise, the second image element becomes a second surface element and the third image element becomes a third surface element

Assigned photoconductor layer. Disadvantage of the tri-level

The method is that only toner particles of two colors in one

Print image can be deposited. A color mix with

The use of three primary colors is therefore ruled out without repeating the process. There are several for color mixing

Printing operations possible, with one printing operation following the steps

Charging, imaging, and developing.

A printer is known from European published patent application EP 0 606 141 A2 which can be used for the electrophotographic printing of a printed image with several colors.

The maximum number of colors with an imagewise exposure step is limited to three colors. For example, it is not possible to print additional spot colors, such as gold or silver, in addition to the three basic colors. In addition, a layer system consisting of four layers is used in a printer according to the European published patent application. In order to produce this layer system, an increased effort is necessary since four layers have to be arranged one above the other instead of two layers.

Furthermore, two light-sensitive layers contained in the layer system must be exposed to two light beams, each of which have different wavelengths. In a first variant of the printer according to the European laid-open publication, the photoconductor is guided twice past an exposure station for image-wise exposure. This reduces the printing speed by half. In both runs, an exposure station must provide two different light beams. Exposure costs are therefore doubled, e.g. with regard to the generation of the exposure signals and with regard to the requirements for the optical system with regard to imaging errors.

In a second variant of the European patent application mentioned, the light-sensitive becomes in one pass

Layer system guided past two exposure points. At each of these exposure points, however, there still have to be two

Exposure beams are provided. The requirements for the imaging accuracy of the optical system used are further increased in the second variant. The different wavelengths of the two exposure beams for the first exposure station thus become TE light waves

(transverse electromagnetic) and m the second

Exposure station TM light waves (transverse magnetic).

The object of the invention is to provide a method for the electrophotographic printing of a print image with at least three Specify colors that allow high quality printing and that can be done on a simple printer.

This object is achieved by a method having the features of patent claim 1. In contrast to the known method from the above-mentioned laid-open publication EP 0 606 141 A2, the invention in the printed image contains image elements with at least three different colors of the color particles applied in the development steps. Despite the absence of subtractive color mixing in one printing process in the method according to the invention, additive color mixing can be produced by color particles arranged next to one another. The method according to the invention only requires an imagewise exposure step with a single light frequency and light of a polarization type, so that a simple imagewise exposure is carried out. The light-sensitive layer system can be simply constructed in the method according to the invention. Compared to document EP 0 606 141 A2, a photoconductor layer and an intermediate layer can be saved when printing at least three colors. A subtractive color mixing can take place in that a further print image is printed on the print image in a subsequent printing process as precisely as possible. The further print image is generated on the same photoconductor after removal of the first print image or on a further photoconductor.

The invention is based on the knowledge that the printing quality decreases in the case of multiple exposure for printing a printed image, since unavoidable imaging errors and a positioning of the layer system which is subject to tolerances cannot guarantee that both exposure steps result in a precise positioning of the picture elements of the Print image. Therefore, only an imagewise exposure step is carried out in the invention. In addition, imaging errors are minimized in the invention by that the light-sensitive layer system used only contains an electrode layer which carries a predetermined reference potential and a photoconductor layer which are mechanically and electrically connected over a large area, for example.

In a first exemplary embodiment of the invention, the printed image contains at least a first picture element of a first color, at least a second picture element with the color of the carrier, at least a third picture element of a third color and at least a fourth picture element of a fourth color. Additive color mixing with three colors is therefore also possible, in particular if a white carrier is used. The second image element can be dispensed with in the rare case in which all image elements of the printed image are covered with color particles. In this case, all measures relating to the second picture element or a second surface element, which are mentioned below, are omitted.

A first surface element is assigned to the first image element, a second surface element to the second image element, a third surface element to the third image element and a fourth surface element of the photoconductor layer to the fourth image element. Since the surface elements to form image elements of different colors are only exposed once in the invention and the repositioning mentioned above for a second exposure is thus eliminated, it can be ensured that the surface elements are exactly aligned with one another. With three colors, when using colors that are sufficiently far apart in the color space, an additive color mixing of many other colors can be carried out. For example, red can be used as the first color, blue as the second color and green as the third color. Accurate position means that adjacent surface elements do not overlap or only overlap to an insignificant extent and that no or almost no empty spaces arise between adjacent surface elements. By covering surface elements, the image elements are assigned to different colors, unwanted toner deposits occur, which lead to a poor print image. The color of the carrier material is unintentionally visible through spaces between image elements, which also results in a poor printed image. In the invention, the unique image-like exposure avoids covering and creating gaps with great accuracy. The result is a high level of pressure.

In the first exemplary embodiment of the invention, the flat elements are exposed differently after a previous charge to an initial potential such that, after exposure, the fourth flat element has a fourth potential, the third flat element has a third potential which is higher in magnitude than the fourth potential, and the second flat element one compared to that third potential in terms of magnitude higher and the first surface element has a first potential in terms of magnitude higher than the second potential. This different exposure is also referred to as imagewise exposure. This gradation of potential means that each color is assigned exactly one potential value. Another imagewise exposure step, in which flat elements are irradiated with different light energies, can be omitted, since after a single imagewise exposure step there is already a clear association between potential values and colors.

After the image-wise exposure, the flat elements with color particles of the first color are developed in a first development step in the first exemplary embodiment of the invention. Color particles of the first color are only deposited on the first flat elements. No toner particles are deposited on the other flat elements. At the time of this development step, the first flat elements have the greatest potential in terms of amount. Be that used development method for applying the color particles of the first color is a development of charged area elements (charged area development). In the first exemplary embodiment of the invention, the color particles of the first color are charged positively in order to facilitate or enable selective deposition on the first flat elements.

In a second development step, in the first exemplary embodiment of the invention, the flat elements are developed with color particles of the fourth color. Negatively charged color particles of the fourth color are deposited on the fourth flat elements. At the time of this development step, the fourth flat elements have the smallest potential in terms of amount. The second development step is accordingly a development of discharged area development.

After the first two development steps, the flat elements in the first exemplary embodiment of the invention are arranged near a light source with approximately uniform light distribution. The arrangement can be achieved, for example, by moving the flat elements past the light source or by moving the light source past the flat elements. However, it is also possible to arrange the flat elements statically with respect to a light source with homogeneous light distribution. In this case, either the flat elements of the layer system assigned to the printed image are arranged at the same time opposite the light source, or the flat elements are arranged one after the other opposite the light source, it being possible for example to expose flat elements which are assigned to image elements of one line at the same time. The invention and the first exemplary embodiment are based on the knowledge that further color camels of further colors can be deposited if similar potential relationships are created as before the second development step. By depositing the negatively charged color particles in the second development step, the potential on the fourth surface elements increases in amount. In the invention, the first surface element covered with color particles and the fourth surface element covered with color particles are exposed to light considerably less than the surface elements not covered, since the light does not penetrate through the color particles, or only weakly. The potential on the uncovered second surface element and on the uncovered third surface element is, however, reduced in amount, since the incident light energy is not absorbed by color particles. The amount of the potential on the third surface element after exposure with the same light energy is lower than the instantaneous potential on the fourth surface element. Accordingly, there are now similar potential relationships for the second area element as they existed for the third area element before the second development step. In a third development step, color particles of the third color are deposited on the third surface elements in the invention. No color particles are deposited on the second surface elements. When the color particles deposited on the other surface elements are transferred to the carrier in a later process step, the carrier remains free of color particles in areas which are assigned to the second surface elements during the transfer. As a result, the printed image ultimately has picture elements with the color of the support.

In a further exemplary embodiment, the printed image contains at least one further image element of a further color. During the imagewise exposure, a further potential is generated on the further surface element, which lies between the first and the third potential or between the second and the third potential. By repeatedly arranging the layer system near the light source with approximately uniform light distribution or near other light sources, the further potential is gradually reduced until it is lower in amount than the instantaneous potentials on the surface elements already covered with color particles. Is this When the condition is reached, color particles of the further color can be applied to the further flat element in a further development step. In the case of the invention, the possible number of different image elements in different colors in a print image is only limited by the height of the initial potential, since the potentials which are assigned to the individual colors should be at least about 300 V apart.

If, in another exemplary embodiment of the invention, after the application of negatively charged color particles of the respective color to at least one of the flat elements, the potential on this flat element is increased in amount, the light source with the uniform light distribution can emit less light energy, since the potentials are reduced only by a smaller amount Amount must be reduced. As a result, the contrast range predetermined by the initial potential can be exploited well. However, there is no need to increase the potential if the potential has already been sufficiently increased in terms of amount by depositing the negatively charged color particles or if the available contrast range allows a high reduction in potential.

The invention also relates to a method in which a positive starting potential is used instead of the negative starting potential, the respective instantaneous potentials on the flat elements having a positive sign instead of the negative sign. In addition, negatively charged color particles are used instead of the positively charged color particles and positively charged color particles are used instead of the negatively charged color particles. The invention thus relates to two potential curves on the flat elements, which differ only in the sign of the potentials. The technical effects are the same for both potential courses. In one exemplary embodiment of the invention, the carrier can be printed directly or, in another exemplary embodiment, indirectly using an intermediate carrier, from which the color particles are transferred to the carrier. The light-sensitive layer system can be protected by using an intermediate carrier, since the material of the intermediate carrier can be selected so that there is minimal mechanical stress on the surface of the photoconductor layer when the intermediate carrier and the layer system come into contact. Sheet-like material or continuous paper is used as the carrier.

The invention also relates to an electrophotographic printer with the features of claim 13. The above-mentioned effects with regard to the method also apply to the printer according to the invention. The printer according to the invention is distinguished from the printer described in the aforementioned European publication in addition to the properties mentioned by a simple structure. In particular, the layer system is made up of only two layers and only one image-wise exposure step per printed image is necessary, so that only one image-wise exposure unit with a simple control is required.

The invention can be carried out with a dry toner containing only solid color particles or with a liquid toner, e.g. the color particles are contained in a liquid.

The invention is described below with the aid of exemplary embodiments. Show:

FIG. 1 shows a schematic representation of an electrophotographic printer with essential electronic and mechanical

Functional units, FIG. 2 shows the printing unit of the printer with essential functional components,

FIG. 3 shows the potential profile on the photoconductor in one exposure step and two toner polatates,

Figure 4 shows the state of flat elements of the

Photoconductor in various process steps, and

Figure 5 shows a second potential curve on the

Photoconductor in one exposure step and two toner polarities.

FIG. 1 shows an outline representation of an electrophotographic printer 10 for carrying out an exemplary embodiment of the method according to the invention. The printer 10 has a transport device 16, which is driven by a motor 12 and a shaft 14, for transporting an endless carrier material 18 past a printing unit 20 essentially in accordance with a predetermined printing speed VD. As an alternative to the endless carrier material 18, single sheets can also be printed in the event of a changed transport. The printing unit 20 forms a multicolored toner image, e.g. is transferred to the carrier material 18 with the aid of a corona device (see FIG. 2).

After the carrier material 18 has been transported past the printing unit 20 m in the direction of an arrow 22 which indicates the transport direction, it is fed to a fixing station 24, in which the toner image which can still be blurred is fused to the carrier material 18 with the aid of pressure and temperature. A first deflection unit 26, which feeds the carrier material 18 to the printing unit 20, is arranged in front of the printing unit 20 in the transport direction 22. Another Umlenkemheit 28 stacks the printed Carrier material 18 onto a stack 30. The carrier material 18 is removed from a stack 32 by the first deflecting unit 26 at the beginning of the printing process. Instead of the two stacks 30 and 32, rollers are also used, on which the carrier material 18 is rolled up.

The printing process is controlled by a print controller 34, which contains at least one microprocessor 36 and a memory 38. The microprocessor 36 processes a print program stored in the memory 38 and controls the printing process. In addition, the print controller 34 also processes image data stored in the memory 38 and transmits the processed image data to the printing unit 20 via a control and data bus 40. The motor 12 is controlled by the print controller 34 via a control line 42 so that the carrier material 18 is one Transport speed has, which corresponds essentially to the printing speed VD. The pressure controller 34 is connected via data lines 44 to an em / output device 46, via which i.a. Operator commands to start the printing process can be entered by an operator.

FIG. 2 shows the printing unit 20 of the printer 10 with essential functional components. The printing unit 20 contains a photoconductor 60 which consists of a flexible layer system which is guided around two deflection rollers 62 and 64 in the manner of a conveyor belt. The deflection roller 64 is driven by a drive motor, not shown, which is controlled by the pressure controller 34 and via the control and data bus 40. The printing unit 20 is surrounded by an opaque chassis 66 made of a stable material. The chassis 66 has an opening 68, through which the photoconductor 60 is guided inside the printing unit 20. Outside of the printing unit 20, the carrier material 18 is guided past the opening 68. No light can strike the photoconductor 60 from the outside through the opening 68, since the entire printer 10 is opaque Has disguise. A corona device 70 is arranged opposite the opening 68, with which a toner image located on the photoconductor 60 is transferred to the carrier material 18. The corona device 70 is also referred to as a transfer printing device.

The photoconductor 60 contains an electrode layer 72 carrying zero potential and an approximately parallel photoconductor layer 74 which is in mechanical and electrical contact with the electrode layer 72 over a large area. The photoconductor 60 is moved by the deflection rollers 62, 64 in the direction of an arrow 76. A surface strip of the photoconductor 60 lying transversely to the transport direction of the photoconductor 60 is successively applied to a charging device 78, a character generator 80, a developer station 82 for depositing black toner particles, a developer station 84 for depositing blue toner particles, a charging device 86, a total exposure unit 88, a developer station 90 for depositing red toner particles, a reloading station 92, the corona device 70, an erasing device 94 and a cleaning device 96.

The charging device 78 contains a corona device which is arranged transversely to the transport direction 76 and charges a surface strip of the photoconductor 60 which is in each case transverse to the transport direction 76 and which is located in the immediate vicinity of the charging device 78, in such a way that an initial potential VA of approximately -1200 V on the surface the photoconductor layer 74 is formed in the area of the surface strip (cf. FIG. 3, step S1).

The character generator 80 contains a row of light-emitting diodes arranged transversely to the transport direction 76, each of which illuminates an area of the photoconductor 60 lying transversely to the transport direction 76. The character generator 80 is controlled by the print controller 34 in such a way that image signals in each case form image elements of a line of the print image at the same time m light signals of the light emitting diodes are implemented. The exposure of the photoconductor 60 increases the potential on the exposed surface elements of the photoconductor 60, since the photoconductor 60 m conducts the exposed areas better, as a result of which charge carriers can flow from the surface of the photoconductor layer 74 to the electrode layer 72 in the region of the exposed surfaces. Flat elements on which black toner particles are to be deposited are not exposed; Flat elements on which no toner particles are to be deposited are exposed to a first light energy; Flat elements on which red toner particles are to be deposited are exposed with a second light energy which is higher than the first light energy: and flat elements on which later blue toner particles are to be deposited are exposed with a third light energy which is higher than the second light energy. With increasing light energy, the potential on the respective flat elements increases, ie the potential changes in a positive direction (cf. FIG. 3, step S2).

The developer station 82 deposits positively charged black K color particles using an auxiliary electrode 120 with a potential VBIAS3 on flat elements which have not been exposed. The exact mechanism of action is explained below with reference to FIG. 3 (step S3).

The developer station 84 deposits negatively charged toner particles of the color blue B with the aid of an auxiliary electrode 122 with a potential VBIAS4 on flat elements which have been exposed to the third light energy. The exact mode of operation of the developer station 84 is also explained below with reference to FIG. 3 (step S4).

By depositing the negatively charged blue toner particles, the potential on the flat elements which have been exposed with the third light energy is reduced again, ie changed in the negative direction. In order to lower the potential on these surface elements even further, ie to change them in the negative direction, the photoconductor 60 is guided past the charging device 86. The charging device 86 contains a corona wire stretched transversely to the transport direction 76, which has a potential which causes the surface of the photoconductor layer 74 to be charged to a potential VB5 in the region of the surface elements covered with blue toner particles. The potential VB5 is smaller in magnitude than an instantaneous potential VR5 on the surface elements that were exposed with the second light energy (cf. FIG. 3, step S5).

The strip of photoconductor 60 under consideration is then guided past total exposure unit 88. The total exposure unit 88 contains a laser diode, which radiates light energy in a glass fiber array arranged transversely to the transport direction 76 of the photoconductor 60. The glass fiber array is designed such that essentially the same light energy is emitted over its entire length. The light of the total exposure unit 88 cannot radiate through already deposited black or blue toner particles because it is absorbed by these toner particles. However, if the light of the total exposure unit 88 hits surface elements of the photoconductor layer 74 which are not yet covered with toner particles, the potential on these surface elements is increased, i.e. changed in the positive direction (see FIG. 3, step S6).

The developer station 90 deposits negatively charged toner particles of the color red R on the surface elements of the photoconductor layer 74 exposed to the second light energy. An auxiliary electrode 124 with a potential VBIAS7 is used. The exact mode of action of the deposition of the red toner particles is also explained below with reference to FIG. 3 (step S7). In the transfer station 92, the positively charged black toner particles are reversed so that almost all of the toner particles deposited on the photoconductor 60 are negatively charged. In the process, a charge is transferred to all surface elements of the photoconductor, as a result of which the potentials on the surface elements decrease, ie change in the negative direction (cf. FIG. 3, step S8). This measure ensures that the transfer of the toner image from the photoconductor 60 to the carrier material 18 is carried out safely with the aid of the corona device 70 (cf. FIG. 3, step S9).

After the transfer of the toner image with the aid of the corona device 70, the photoconductor 60, which is now essentially free of toner particles, is guided past the erasing device 94. The erasing device 94 contains a corona device 98 and an exposure unit 100, by means of which residual charges present on the photoconductor 60 are removed.

Toner particles that remain on the photoconductor 60 after the transfer of the toner image are removed from the photoconductor 60 in the cleaning device 96 with the aid of a brush 102. After being transported past the cleaning device 96, the strip of the photo conductor 60 under consideration is again in a clean initial state and has approximately the same potential at all points.

FIG. 3 shows the potential profile on the surface of the strip of the photoconductor 60 under consideration in one exposure step and two toner polarities. The time is plotted on the abscissa axis, which is divided into nine successive time steps S1 to S9. The potential on the surface of the photoconductor layer 74 with respect to the potential on the electrode layer 72 is shown on the ordinate axis. In step S1, the potential on the surface of the photoconductor layer 74 is shifted by the action of the charging device 78 in the negative direction to the initial potential VA, which, as already mentioned, has the value of -1200 V.

In step S2, the image-wise exposure takes place with the aid of the character generator 80, as a result of which the potential curve shown on the surface of selected surface elements of the photoconductor layer 74 is established. Surface elements that are later to be covered with black toner particles are not exposed. The potential VA shifts on these surface elements only slightly in the positive direction in the course of step S2 due to a self-discharge of the photoconductor 60 that cannot be suppressed to a value VK2. The potential on the surface elements that are exposed with the first light energy changes in the positive direction to a value VW2 of approximately -800 V. The potential on the surface elements that are exposed with the second light energy changes in the course of step S2 in a positive direction to a potential value VR2 of approximately -400 V. The potential on the surface elements which were exposed with the third light energy changes in step S2 in a positive direction approximately to a potential value VB2 of approximately -100 V.

In step S3, positive black toner particles are deposited by the developer station 82. The auxiliary electrode 120 in the vicinity of the photoconductor 60 has the auxiliary potential VBIAS3 of approximately -900 V. The positively charged black toner particles are located on the auxiliary electrode 120. Since the potential VBIAS3 is lower than the potentials VW2, VR2 and VB2, these potentials are positive with respect to the potential VBIAS3. However, the positively charged black toner particles can only be deposited on an area which has a lower potential than the potential VBIAS3. This only applies to surface elements that were not exposed in step S2. As a result, the are on these surface elements black toner particles. By depositing the positively charged toner particles, the potential on the surface elements covered with black toner particles increases to a potential value VK3. As a result of the self-discharge of the photoconductor 60 which cannot be avoided, the potentials VW2, VR2 and VB2 increase slightly to the potential values VW3, VR3 and VB3.

In step S4, negative blue toner particles are deposited by the developer station 84. The auxiliary electrode 122 in the immediate vicinity of the photoconductor 60 has the auxiliary potential VBIAS4 of approximately -390V. The negatively charged blue toner particles are located on the auxiliary electrode 122. Since the potential VBIAS4 is higher than the potentials VK3, VW3 and VR3, these potentials lie with respect to the potential VBIAS4 in the negative direction. However, the negatively charged blue toner particles can only be deposited on a surface that is higher in potential VBIAS4, i.e. potential shifted in the positive direction. This only applies to flat elements that were exposed to the third light energy in step S2. As a result, the blue toner particles are deposited on these flat elements. By depositing the negatively charged blue toner particles, the potential on the surface elements covered with blue toner particles is reduced to a potential value VB4. Due to the self-discharge of the photoconductor 60, the potentials VK3, VW3 and VR3 increase slightly to the potential values VK4, VW4 and VR4.

In step S5, the potential VB4 on the surface of the flat elements covered with blue toner particles is reduced to approximately -390 V with the aid of the charging device 86, ie shifted in the negative potential direction. The self-discharge of the photoconductor 60 increases the potentials VK, VW4 and VR4 in step S5 to the potentials VK5, VW5 and VR5. In step S6, the potential VW5 or VR5 on the surface elements not covered with toner particles is increased by approximately 400 V to the potentials VW6 or VR6 by the light emitted by the total exposure unit 88. The potential on surface elements which were exposed with the second light energy in step S2 becomes the highest potential on one of the surface elements in step S6 due to the further exposure in step S6. The potentials VK5 and VB5 increase slightly due to the self-discharge of the photoconductor 60 to the potentials VK6 and VB6. There is a difference of approximately 400 V between the potentials VR6 and VB6, so that in the following step S7, similar to step S4, toner particles can be deposited on the surface elements which were exposed in step S2 with the second light energy.

In step S7, negative red toner particles are deposited by the developer station 90. The auxiliary electrode 124 in the immediate vicinity of the photoconductor 60 has the auxiliary potential VBIAS7 of approximately -370 V. The negatively charged red toner particles are located on the auxiliary electrode 124. Analogous to the electrical conditions described in step S4, the negative toner particles are deposited on the surface elements which were exposed in step S2 with the second light energy. The potentials VK6, VW6 and VB6 increase due to the self-discharge of the photoconductor 60 to the potential values VK7, VW7 and VB7.

In step S8, the strip of photoconductor 60 under consideration is guided past the transfer station 92. The transfer station 92 contains a corona device which transfers the potential on the surface of the photoconductor layer 74. When it is transported past, the potentials on all surface elements are significantly reduced, the polarity of the black toner particles on the photoconductor 60 being reversed, so that the black toner particles are also negatively charged. In step S9, through the action of the positively charged corona device 70, the toner particles of surface elements covered with toner particles are transferred onto the carrier material 18 essentially while maintaining their position relative to one another. The potential on the surface elements of the photoconductor 60 increases to approximately -400 V.

In a step, not shown, the remaining charge on the photoconductor 60 is removed by the quenching device 94, so that the photoconductor 60 has a potential value of approximately 0 V on its surface after passing through the quenching device 94.

FIG. 4 shows the state of surface elements of the photoconductor 60 at the end of steps S1 to S9. Part a of FIG. 4 shows a printed image 140 which contains four image elements 142 to 148. The picture element 142 has the color blue B, which is represented in FIG. 4 by a horizontal hatching.

The picture element 144 has the color red R, which is represented in FIG. 4 by vertical hatching. The image element 146 has the color black K, which is represented in FIG. 4 by an inclined hatching, the hatching lines of which are arranged at approximately 45 ° with respect to the horizontal. The image element 148 has the color white W (color of the carrier material 18), which is represented in FIG. 4 by hatching, the hatching lines of which are aligned approximately at an angle of 135 ° with respect to the horizontal.

Part b shows a strip-shaped section 150 of the photoconductor 60. The section 150 is arranged on the photoconductor 60 transversely to the transport direction 76. In FIG. 4, section 150 is shown in a top view, with photoconductor layer 74 on top. The print control 34 assigns surface elements 152 to 158 on the surface of the photoconductor 60 to the image elements 142 to 148. The surface element 152 is assigned to the image element 142. The visual Element 144, 146 and 148 are assigned to surface element 154, 156 and 158, respectively. The assignment takes place in such a way that adjacent picture elements of the printed image 140 are also assigned to adjacent area elements. In step S1, the initial potential VA is generated by the charging device 78 on each of the surface elements 152 to 158.

Part c of FIG. 4 shows the state of the flat elements 152 to 158 after the image-wise exposure in step S2. Since the largest, third light energy falls on the flat element 152, a charge equalization takes place via the photoconductor layer 74, which conducts light well in the area of the flat element 152, as a result of which the potential VB2 is established on the surface of the flat element 152. The surface element 154 is exposed to the second light energy, which is lower than the third light energy. As a result, the potential VR2, which is lower than the potential VB2, is established on the surface of the flat element 154. The flat element 156 is not illuminated during image-wise exposure. As a result, a potential VK2 is established on the surface of the surface element 156 at the end of the exposure step S2, which is only slightly above the initial potential VA. The potential VW2 is set on the surface of the flat element 158 after exposure with the first light energy in step S2. Since the first light energy is lower than the second light energy, the potential VW2 is lower than the potential VR2.

A flat element which is not covered with toner particles and which has the highest potential at the end of one of the steps S2 to S9 is identified by an asterisk in the upper right corner of the respective flat element. A flat element which is not covered with toner particles and which has the lowest potential value at the end of one of the steps S2 to S9 is identified by a cross in the upper right corner of the relevant flat element. In part c of Figure 4 the flat element 152 the greatest potential and the flat element 156 the smallest potential.

Part d of FIG. 4 shows the surface potentials on the surface elements 152 to 158 at the end of step S3. During step S3, section 150 is transported past developer station 82. For the reasons mentioned above, black toner particles only accumulate on the surface of the flat element 156, so that this flat element is completely covered with black toner particles (45 ° hatching).

Part e of FIG. 4 shows the flat elements 152 to 158 at the end of step S4. In step S4, section 150 is transported past developer station 84. For the reasons mentioned above, blue toner particles are deposited on the flat element 152 (horizontal hatching), so that both the flat element 152 and the flat element 156 are now covered with toner particles.

Part f of FIG. 4 shows the flat elements 152 to 158 at the end of step S6, to which the section 150 has been exposed uniformly. The uniform exposure leads to an increase in potential on the surface of the flat elements 154 and 158, which are not covered with toner particles, since the incident light reduces the resistance of the photoconductor layer 74 and partial charge carrier compensation between charge carriers on the surface of these flat elements and charge carriers the electrode layer 72 takes place. At the end of step S6, the flat element 154 has the greatest potential on its surface.

Part g of FIG. 4 shows the flat elements 152 to 158 at the end of step S7. In the course of this step, section 150 is transported past developer station 90. For the reasons mentioned above, red toner particles are deposited on the flat element 154 (vertical hatching). The surface elements 152 to 156 are thus covered with toner particles.

Part h of FIG. 4 shows a section 160 of the carrier material 18 at the end of step S9. In step S9, the toner particles on the section 150 are transferred onto the section 160 of the carrier material 18 essentially while maintaining their mutual position. As already mentioned, the carrier material 18 has the color white W (135 ° hatching), so that as a result of the method described, the printed image 140 with the image elements 142 to 148 was printed on the section 160 of the carrier material 18.

A picture element has e.g. when printing with the printer 10 with a resolution of 600 pixels per 25.4 mm, a width of about 0.042 mm, so that the representations in FIG. 4 are a strong enlargement with a magnification factor of about 200. The human eye can no longer resolve the pixels individually at a normal reading distance of approximately 30 cm. This results in color mixing effects. The blue picture element 142 and the red picture element 144 result e.g. the mixed color perceived by the eye violet.

The process described for three colors leads to a process with n colors, in that the initial potential VA is selected to be approximately equal to the n-fold potential requirement for a single development step. In addition, at least n different light energies must be able to be generated per picture element in the case of imagewise exposure, so that n + 1 different potentials can be generated. Steps S5 to S7 are repeated n-3 times after step S7. The letter n is a natural number that can take the values 4, 5, etc.

FIG. 5 shows a second potential profile on the surface of surface elements of the photoconductor 60 in one exposure step and two toner polarities. The The potential curve shown applies to a printer which contains a printing unit 20 '(not shown) which differs from the printing unit 20 in that the corona device 70, the charging device 78, the charging device 86, the transfer station 92 and the corona device 98 are operated with the opposite operating voltage . In addition, a developer station is used instead of the developer station 82, which deposits negative toner particles of the color black with the aid of an auxiliary electrode with the potential VBIAS3 'of approximately +900 V. Instead of the developer station 84, a developer station for positively charged blue toner particles is used. The auxiliary electrode when the blue toner particles are deposited has an auxiliary potential VBIAS4 ¹ of approximately +390 V. Instead of the developer station 90, a developer station for positively charged red toner particles is used. When the red toner particles are deposited, an auxiliary electrode with an auxiliary potential VBIAS7 'of approximately +370 V is used.

The potential curve shown in FIG. 5 differs from the potential curve in FIG. 3 in that the signs of the potentials are reversed compared to FIG. 3. Taking into account the signs, the statements made with reference to FIG. 3 also apply to the potential profile of FIG. 5. Instead of steps S1 to S9, steps S1 'to S9' are now used. Instead of the potential VA, a potential VA 'with opposite signs is used in FIG. In addition, changed potentials VK2 'to VK7', VW2 'to VW7 \ VR2' to VR7 'or VB2' to VB7 'replace the potentials VK2 to VK7, VW2 to VW7, VR2 to VR7 and VB2 to VB7 .

Claims

claims

1. Method for the electrophotographic printing of a printed image (140) with several colors on a carrier (18),

in which a layer system (60) is charged to an initial potential,

at least three different potentials are generated on the layer system (60) by imagewise exposure,

wherein a third potential has a larger value than a fourth potential and a first potential has a larger value than the third potential,

in a development step with color particles of a first color, these color particles are applied at locations with the first potential,

in a development step with color particles of a fourth color, these are applied at locations with the fourth potential,

the third potential is reduced in magnitude by uniform exposure to a value below the instantaneous value of the fourth potential, which is present with color particles of the fourth color after the development step mentioned,

and in a development step with color particles of a third color, these color particles are applied at locations with the lowered third potential, characterized in that the printed image (140) contains at least one image element of the first color, the fourth color and the third color.

Method according to Claim 1, characterized in that the printed image (140) has at least one first image element (146) of a first color (K), at least one second image element (148) with the color (W) of the carrier (18), at least one third Contains picture element (144) of a third color (R) and at least em fourth picture element (142) of a fourth color (B),

the first picture element (146) and a first flat element (156) are assigned to a photoconductor layer (74), the second picture element (148), a second flat element (158), the third picture element (144), a third flat element (154) and the fourth picture element ( 142) a fourth flat element (152) is assigned to the photoconductor layer (74),

the photoconductor layer (74) and a predetermined reference potential leading electrode layer (72) are contained in a layer system (60),

and that the following steps are carried out:

Sl) the flat elements (152 to 158) are charged to a negative initial potential (VA) (step S1),

S2) the flat elements (152 to 158) are exposed differently in such a way that after the exposure the fourth flat element (152) has a fourth potential (VB2), the third flat element (154) e has a larger third potential than the fourth potential (VB2) (VR2), the second surface element

(158) em higher than the third potential (VR2) in terms of amount, second potential (VW2) and that first surface element (156) has a higher first potential (VK2) than the second potential (VW2) (step S2),

S3) the flat elements (152 to 158) are developed with color particles of the first color (K) (step S3),

wherein positively charged color particles of the first color (K) are deposited on the first flat element (156) using a first auxiliary electrode (120) which has a first auxiliary potential (VBIAS3) which is higher in amount than the instantaneous potential (VW3) the second flat element (158) and lower in amount than the instantaneous potential (VK3) on the first flat element (156),

S4) the flat elements (152 to 158) are developed with color particles of the fourth color (B) (step S4),

wherein negatively charged color particles of the fourth color (B) are deposited on the fourth flat element (152) using a second auxiliary electrode (122) which has a second auxiliary potential (VBIAS4) which is higher in amount than the current potential (VB4) the fourth flat element (152) and is smaller in amount than the instantaneous potential (VR4) on the third flat element (154),

S6) the flat elements (152 to 158) are arranged near a light source (88) with an approximately uniform light distribution (step S6),

wherein the first flat element (156) covered with color particles and the fourth flat element (152) covered with color particles are exposed to light considerably less than the second flat element which is not covered. ment (158) and the uncovered third surface element (154), and wherein the current potential (VR6) on the third surface element (154) is reduced in amount to em potential (VR6), which is lower in quantity than the current potential (VB6 ) on the fourth surface element (152),

S7) the flat elements (152 to 158) are developed with color particles of the third color (R) (step S7),

wherein negatively charged color particles of the third color (R) are deposited on the third surface element (154) using a third auxiliary electrode (124) which has a third auxiliary potential (VBIAS7) which is higher in amount than the current potential (VR7) the third area element (154) and is smaller in amount than the current potential (VB7) on the fourth area element (152) and as the current potential (VW7) on the second area element (158).

3. The method according to claim 1, characterized in that the printed image (140) at least em first picture element (146) of a first color (K), at least em third picture element (144) of a third color (R) and at least a fourth picture element (142 ) contains a fourth color (B),

the first picture element (146) is assigned a first flat element (156) of a photoconductor layer (74), the third picture element (144) is a third flat element (154) and the fourth picture element (142) is a fourth flat element (152) of the photoconductor layer (74) assigned, the photoconductor layer (74) and an electrode layer (72) carrying a predetermined reference potential are contained in a layer system (60),

and that the following steps are carried out:

Sl) the surface elements (152 to 156) are loaded to a negative initial potential (VA) (step S1),

S2) the surface elements (152 to 156) are exposed differently in such a way that after exposure the fourth surface element (152) has a fourth potential (VB2), the third surface element (154) has a third potential which is higher in magnitude than the fourth potential (VB2) (VR2), and the first surface element

(156) has a higher first potential (VK2) than the third potential (VR2) (step S2),

S3) the surface elements (152 to 156) are developed with color particles of the first color (K) (step S3),

wherein positively charged color particles of the first color (K) are deposited on the first surface element (156) using a first auxiliary electrode (120) which has a first auxiliary potential (VBIAS3) which is higher in amount than the instantaneous potential (VR3) the third surface element (154) and is lower in amount than the instantaneous potential (VK3) on the first surface element (156),

S4) the surface elements (152 to 156) are developed with color particles of the fourth color (B) (step S4),

wherein negatively charged color particles of the fourth color (B) are deposited on the fourth surface element (152) using a second auxiliary electrode (122). which has a second auxiliary potential (VBIAS4) which is higher in amount than the current potential (VB4) on the fourth surface element (152) and lower in quantity than the current potential (VR4) on the third surface element (154),

S6) the flat elements (152 to 156) are arranged near a light source (88) with approximately uniform light distribution (step S6),

wherein the first flat element (156) covered with color particles and the fourth flat element (152) covered with color particles are exposed to considerably less light than the uncovered α third third element (154),

and wherein the current potential (VR6) on the third surface element (154) is reduced in magnitude to the potential (VR6), which is smaller in amount than the current potential (VB6) on the fourth

Flat element (152),

S7) the flat elements (152 to 156) are developed with color particles of the third color (R) (step S7),

wherein negatively charged color particles of the third color (R) are deposited on the third surface element (154) using a third auxiliary electrode (124) which has a third auxiliary potential (VBIAS7) which is higher in magnitude than the current potential (VR7) on the third Flat element (154) and lower in amount than the instantaneous potential (VB7) on the fourth flat element (152).

Method according to Claim 2 or 3, characterized in that the printed image (140) contains at least one further image element of a further color, the further image element is assigned to a further flat element of the photoconductor layer (74),

the further flat element is loaded to the initial potential (VA) (step S1),

the further flat element is exposed during the different exposure in such a way that after exposure it has a further potential which is higher in amount than the third potential (VR2) and lower in amount than the second potential (VW2) or the first potential (VK2) ( Step S2),

that if the flat elements (152 to 158) are arranged several times near the light source (88) or near a respective light source, the further flat element which is not covered is exposed to considerably more light than flat elements covered with color particles,

the potential on the further flat element being reduced in magnitude when the respective arrangement is carried out,

and that the flat elements are developed with color particles of the other color,

wherein negatively charged color particles of the further color are deposited on the further flat element using a further auxiliary electrode which has a further auxiliary potential which is higher in amount than the instantaneous potential on the further flat element and lower in amount than the instantaneous potentials on the other flat elements .

Method according to one of claims 1 to 4, characterized in that the deposited color particles positive ver polarity can be reloaded to a negative polarity (step S8).

6. The method according to claim 1, characterized in that the printed image (140) at least a first picture element

(146) a first color (K), at least a second

Picture element (148) with the color (W) of the carrier (18), at least one third picture element (144) of a third

Contains color (R) and at least one fourth picture element (142) of a fourth color (B),

a first surface element (156) of a photoconductor layer (74) is assigned to the first image element (146), a second surface element (158) to the second image element (148), a third surface element (154) to the third image element (144) and the fourth image element ( 142) a fourth surface element (152) is assigned to the photoconductor layer (74),

the photoconductor layer (74) and an electrode layer (72) carrying a predetermined reference potential are contained in a layer system (60),

and that the following steps are carried out:

51) the surface elements (152 to 158) are charged to a positive initial potential (VA ') (step S1'),

52) the surface elements (152 to 158) are exposed differently in such a way that after exposure the fourth surface element (152) has a fourth potential (VB2 '), the third surface element (154) has a fourth potential (VB2') third potential (VR2 '), the second surface element (158) is higher than the third potential

(VR2 ') higher second potential (VW2') and the first surface element (156) a compared to the second potential (VW2 ') has higher absolute value (VK2') (step S2 '),

53) the flat elements (152 to 158) are developed with color particles of the first color (K) (step S3 '),

wherein negatively charged color particles of the first color (K) are deposited on the first flat element (156) using a first auxiliary electrode (120) which has a first auxiliary potential (VBIAS3 '), which is higher in amount than the current potential (VW3') the second flat element (158) and is smaller in amount than the instantaneous potential (VK3 ') on the first flat element (156),

54) the flat elements (152 to 158) are developed with color particles of the fourth color (B) (step S4 '),

wherein positively charged color particles of the fourth color (B) are deposited on the fourth flat element (152) using a second auxiliary electrode (122) which has a second auxiliary potential (VBIAS4 ') which is higher in amount than the instantaneous potential (VB4' ) on the fourth surface element (152) and in terms of magnitude lower than the instantaneous potential (VR ') on the third surface element (154),

S6) the flat elements (152 to 158) are arranged near a light source (88) with an approximately uniform light distribution (step S6 '),

wherein the first flat element (156) covered with color particles and the fourth flat element (152) covered with color particles are exposed to light considerably less than the second flat element which is not covered. ment (158) and the uncovered third surface element (154), and wherein the current potential (VR6 ') on the third surface element (154) is reduced in amount to a potential (VR6') which is lower in amount than the current potential (VB6 ') on the fourth surface element (152),

S7) the surface elements (152 to 158) are developed with color particles of the third color (R) (step

S7-),

wherein positively charged color particles of the third color (R) are deposited on the third surface element (154) using a third auxiliary electrode (124) which has a third auxiliary potential (VBIAS7 ') which is higher in magnitude than the current potential (VR7') the third surface element (154) and is smaller in amount than the current ^• potential (VB7 ') on the fourth surface element (152) and as the current potential (VW7') on the second surface element (158).

7. The method according to claim 1, characterized in that the printed image (140) at least a first picture element

(146) a first color (K), at least a third

Contains a picture element (144) of a third color (R) and at least a fourth picture element (142) of a fourth color (B),

a first surface element (156) of a photoconductor layer (74) is assigned to the first image element (146), a third surface element (154) to the third image element (144) and a fourth surface element (152) of the photoconductor layer (152) to the fourth image element (142) 74) is assigned, the photoconductor layer (74) and an electrode layer (72) carrying a predetermined reference potential are contained in a layer system (60),

and that the following steps are carried out:

Sl) the surface elements (152 to 156) are loaded to a positive initial potential (VA ') (step Sl'),

S2) the surface elements (152 to 156) are exposed differently in such a way that after exposure, the fourth surface element (152) has a fourth potential (VB2 '), the third surface element (154) has a magnitude higher than the fourth potential (VB2') - Heres third potential (VR2 '), and the first surface element (156) has a higher than the third potential (VR2') in terms of amount first potential (VK2 ') (step S2'),

S3) the surface elements (152 to 156) are developed with color particles of the first color (K) (step S3 '),

wherein negatively charged color particles of the first color (K) are deposited on the first surface element (156) using a first auxiliary electrode (120) which has a first auxiliary potential (VBIAS3 ') which is higher in amount than the instantaneous potential (VR3' ) on the third surface element (154) and lower in amount than the current potential (VK3 ¹ ) on the first surface element (156),

S4) the surface elements (152 to 156) are developed with color particles of the fourth color (B) (step S4 '),

wherein on the fourth surface element (152) positively charged color particles of the fourth color (B) below Can be deposited using a second auxiliary electrode (122) which has a second auxiliary potential (VBIAS4 ') which is higher in magnitude than the current potential (VB4') on the fourth surface element (152) and lower in magnitude than the current potential

(VR4 ') on the third surface element (154),

56) the surface elements (152 to 156) are arranged near a light source (88) with approximately uniform light distribution (step S6 '),

the first surface element (156) covered with color particles and the fourth surface element (152) covered with color particles being exposed to the light considerably less than the third surface element (154) not covered,

and wherein the current potential (VR6 ¹ ) on the third surface element (154) is reduced in amount to a potential (VR6 ') which is lower in quantity than the current potential (VB6') on the fourth surface element (152),

57) the surface elements (152 to 156) are developed with color particles of the third color (R) (step

S7 '),

wherein positively charged color particles of the third color (R) are deposited on the third surface element (154) using a third auxiliary electrode (124) which has a third auxiliary potential (VBIAS7 ') which is higher in amount than the current potential (VR7') the third surface element (154) and is smaller in amount than the current potential (VB7 ') on the fourth surface element (152).

8. The method according to claim 6 or 7, characterized in that the printed image (140) contains at least one further image element of a further color,

the further image element is assigned to a further surface element of the photoconductor layer (74),

the further surface element is loaded to the initial potential (VA ') (step S1'),

the further surface element is exposed during different exposure in such a way that after exposure it has a further potential which is higher in amount than the third potential (VR2 ') and lower in amount than the second potential (VW2') or the first potential (VK2 ' ) is (step S2 *),

that if the surface elements (152 to 158) are arranged several times near the light source (88) or near a respective light source, the further surface element not covered is exposed to considerably more light than surface elements covered with color particles,

the potential on the further surface element being reduced in magnitude when arranged in each case,

and that the surface elements are developed with color particles of the further color,

wherein positively charged color particles of the further color are deposited on the further surface element using a further auxiliary electrode which has a further auxiliary potential which is higher in magnitude than the instantaneous potential on the further surface element and lower in amount than the instantaneous potentials on the other surface elements.

9. The method according to any one of claims 6 to 8, characterized in that the deposited color particles of negative polarity are transferred to a positive polarity (step S8 ').

10. The method according to any one of the preceding claims, characterized in that after the application of charged color particles of the respective color to at least one of the flat elements, the potential on this flat element is increased in amount (step S5, step S5 ').

11. The method according to any one of the preceding claims, characterized in that the deposited color particles are transferred from the photoconductor layer (74) to the carrier (18) substantially while maintaining their mutual position.

12. The method according to any one of claims 1 to 10, characterized in that the deposited color particles are transferred to an intermediate carrier substantially while maintaining their mutual position,

and that the color particles are transferred from the intermediate carrier to the carrier (18) essentially while maintaining their mutual position.

13. Electrophotographic printer (10), in particular for performing the method according to one of claims 1 to 12,

with a light-sensitive layer system (60) which contains an electrode layer (72) carrying a predetermined reference potential and a photoconductor layer (74),

with a charging device (78) for generating an initial potential (VA, VA ') on the photoconductor layer (72), an exposure device (80) for imagewise exposure of the photoconductor layer (72),

a first developer station (82) for depositing color particles of a first polarity and a first color (K) on the layer system (60),

a second developer station (84) for depositing color particles of the other polarity and a second color (B) on the layer system (60),

at least one total exposure unit (88) for uniform exposure (60),

and with at least one further developer station (90) for depositing color particles of the other polarity and a further color (R) on the layer system (60).

14. Printer according to claim 13, characterized by at least one potential increasing device (86) for increasing the amount of only the smallest amount of potential on the layer system (60),

and / or a transfer station (92) for transferring the deposited color particles of the first polarity to the other polarity,

and / or a transfer device (70) for transferring the deposited color particles from the layer system to a carrier (18),

and / or an erasing device (94) for erasing a residual charge image on the layer system (60),

and / or by a cleaning device (96) for cleaning the layer system (60).